Small cryocoolers are important tools to maintain cryogenic environment for industrial applications of superconducting magnet and cryoelectronics. In this lecture, basic understanding of thermodynamic cycles and efficiency, configuration of practical cryocoolers and several comments for users are presented.
A laser Doppler velocimeter (LDV) was applied to the measurement of thermal counterflow jet in He II, which is capable of direct measurement of the normal fluid velocity at the jet nozzle exit. The analysis of velocity measurement suggests that the normal fluid jet entrains the surrounding superfluid, which result in the zero relative velocity of two fluid even very close to the nozzle exit. It is demonstrated that the second sound Helmholtz oscillation measurement is of use as a diagnostics for superfluid turbulent state. The frequency and the decay of the oscillation were investigated for this purpose. It is also suggested that a weak superfluid turbulent state may exist in small heat input case. While, in large heat input case, the turbulent state was identified as the Gorter-Mellink type superfluid turbulent state.
30kA, NbTi Demo Poloidal Coils (DPC-U1, U2) were fabricated and tested in the Demo Poloidal Coil (DPC) project at Japan Atomic Energy Research Institute (JAERI). The DPC-U has large forced-flow cable-in-conduit conductor and its strand surface is insulated by the formvar. The DPC-U exhibited an instability in pulse and even in DC charging, such as the DPC-U had quenches at the 1/10th current of the conductor critical current. Detailed investigations on the quench currents and stability experiments were carried out in order to clarify the cause of the instability. These experimental results showed existence of the non-uniformity of current distribution in the conductor even in DC charging. And it could be concluded the instability was caused by the non-uniform current distribution and long time constant of current redistribution because of insulation between the strands, However, it was also shown the current distribution could be uniform, depositing energy on the conductor by the inductive heater without quench. Therefore it was suggested stable operation of the DPC-U was possible using inductive heater.
In this paper, the phenomena of pool boiling and formation of flow convection in the cooling channels are observed by an optical method. The laser schlieren method with helium-neon laser of 7mW and a high speed camera with maximum shooting speed of 10, 000 frames per second are used to observe the fast phenomena. Analyzing films, transient formations of the flow convection have been observed. A bubble cluster with the thickness of around a hundred μm boundary layer has been formed during first 10ms and after that a turbulent layer has developed. The phenomena of the overshooting of the flow convection have been observed during heating. The flow velocity induced from the elapsing time till the overshooting becomes at maximum is almost equal to the predicted flow velocity calculated from the energy conservation law.
Many efforts have been made to apply high-Tc superconductors to a variety of levitation systems. A levitation mechanism has already been developed by our group, which consists of a superconductor and a set of alternating-polarity magnets. Such levitation mechanisms make it possible to design new actuators. We have made a new actuator which is levitated using the levitation mechanism and driven by a force between electromagnets and superconductors. The actuator consists of a superconductin slider, electromagnets for driving, levitation mechanisms for supporting the slider. The position of the slider is detected by a Hall sensor. Fuzzy control theory is applied to the actuator. This paper describes the characteristics of the superconducting linear actuator and the effectiveness of fuzzy control.